1. Field of the Invention
The present invention relates to the production of hydrogen rich gas by autothermal steam reforming of a hydrocarbon process gas, wherein the hydrogen rich gas being withdrawn from the autothermal steam reforming process at elevated pressure is depressurized in a gas turbine for generation of valuable power.
2. Description of the Related Art
Autothermal steam reforming is a well-known process for the preparation of hydrogen rich synthesis gas. The process is conventionally performed in a combustion reactor with a top combustion zone, and a bottom catalyst zone. In the combustion zone, hydrocarbon feedstock is partially oxidized with an oxygen-containing atmosphere. Partially oxidized effluent from the combustion zone is subsequently subjected to steam reforming in the presence of a steam reforming catalyst arranged in fixed bed manner in the bottom part of the reactor. Typical operation conditions in the above process are 850.degree. C. and 2-4 MPa at steam to carbon ratios above 1, depending on the desired product gas.
A problem with autothermal steam reforming is formation of soot in the substoichiometric combustion of hydrocarbons. In particular, at low steam/carbon ratios in the feed gas to the autothermal reformer, soot is formed markedly in the combustion zone.
In a number of industrial applications, a low steam to carbon ratio is required in the feed gas. Thus, in the manufacture of synthesis gas, steam to carbon ratios below 1 are advantageous to obtain the optimum hydrogen to carbon monoxide ratio in the product gas.
Several attempts to reduce soot formation at low steam to carbon ratios in autothermal reforming have been made in the past, including specific burner designs and control of operation conditions.
In co-pending European Patent Application No. 99102386, a process for soot free autothermal catalytic steam reforming is described, wherein formation of soot is avoided by controlling the operation pressure within a certain range depending on the adiabatic gas temperature of the reformed feed stock and the steam to carbon ratio. It was, furthermore, found that operation pressures above 3.5 MPa allow soot free reforming at a very low steam to carbon ratio.
The disadvantages of operating at high operation pressure in the autothermal steam reforming process are the expense involved in compressing the feed gas and the pressure of produced synthesis gas, which, for applications in subsequent process units, will be typically required at a lower pressure.